It is well known that cellular DNA can be modified by a variety of agents, occasionally resulting in the creation of a new cell species, such as tumor cells. The fact that carcinogenic agents appear to be prevalent in the environment, coupled with the relatively low occurrence of cancer in certain populations, has suggested the presence of major anti-carcinogenic factors in the environment as well.
The multi-stage nature of carcinogenesis has been previously shown for skin cancer (Berenblum, I., Cancer Res., 1, 44, 1941). In this pioneering work, inflammatory croton oil, when painted on the backs of mice, caused tumors only when the mice were pretreated with a minute dose of a carcinogen that by itself would be insufficient to cause cancer. It has been postulated that a carcinogen gives rise to a cell type indistinguishable from normal cells until exposure to a promoting agent converts it to a tumor cell. Many known tumor promoters are derivatives of 12-0-tetradecanoylphorbol-13-acetate (TPA), the parent alcohol of the tumor promoting compounds of croton oil. Other types of tumor promoting agents include teleocidin, aplysiatoxin and okadaic acid.
Current hypotheses recognize three phases in tumor development: Stage 1--initiation, Stage 2--promotion, and Stage 3--progression to malignant neoplasm. In contrast to tumor initiation, multiple applications or prolonged exposure to a tumor promoting agent are believed to be required before tumor growth becomes inevitable. The extended nature of Stage 2 appears to provide an opportunity to prevent tumor formation by utilizing dietary and other components which inhibit the effects of tumor promotion agents.
The mechanism of tumor promotion displays a number of common characteristics among the seemingly unrelated agents which contribute to cancer, including protein kinase C activation, protease activity, induction of oxygen radicals and poly(ADP)ribose polymerase. Thus, as might be expected, numerous agents have been shown to have some activity as anti-tumor promotor agents. Such agents include protease inhibitors, retinoids, sarcophytols and nicotinamides. These agents appear to share two characteristic properties: they suppress oxyradical induction of tumor promoter-induced neutrophils and they prevent oncogene transformation in ras-induced NIH 3T3 cells.
Activated (i.e. mutated) ras genes have been found in 10 to 20% of all human cancers. Over 90% of pancreatic cancers, 50% of colorectal cancers and 30% of lung cancers have activated ras genes. A single topical application of 7,12-dimethylbenz[a]anthracene to mouse skin can induce a mutation in codon 61 of c-H-ras gene in the affected cells.
The ras genes encode guanine nucleotide binding proteins that participate in the control of eukaryotic cell proliferation. Ras proteins associate with the inner surface of the plasma membrane through the farnesyl and palmitoyl groups. Ras proteins have two states: A GTP-bound active state and a GDP-bound inactive state. Active ras protein is converted to an inactive form by an intrinsic guanosine triphosphatase (GTPase) activity that is stimulated by interaction with a GTPase activating protein (GAP). Upon binding GTP, ras proteins become activated and are capable of stimulating cell proliferation. Mutated ras proteins decrease their intrinsic GTPase activity by a conformational change in the guanine nucleotide binding site of the protein or by a nonproductive association with GAP protein. Therefore, the mutated ras protein remains in the active conformation longer (Schafer, W. R. et al., Nature, 28, 379 (1989)).
However, compounds presently known to be useful as anti-tumor promoters typically have one or more drawbacks, including low activity, high cost of manufacture and toxicity. Thus it is considered desireable to develop a highly active antitumor promoter compound which is relatively nontoxic and inexpensive to manufacture.